1. Field of the Invention
The present invention relates to a photovoltaic device and a method of manufacturing thereof. More specifically, the present invention relates to a so-called series type photovoltaic device wherein a plurality of series connected photoelectric conversion cells composed of a semiconductor layer such as amorphous silicon are arranged in a direction of width thereof on a single substrate, and a method of manufacturing thereof.
2. Description of the Prior Art
This kind of photovoltaic device is disclosed, for example, in U.S. Pat. No. 4,281,208, assigned to the same assignee as the present invention. A brief description will be given here of the structure of this photovoltaic device shown in FIG. 1 within the context required for understanding the present invention.
A plurality of photoelectric conversion cells 12a, 12b, 12c, are formed on a glass substrate 10. Transparent electrodes 14a, 14b, 14c, are formed with a constant interval between adjacent photoelectric conversion cells 12a, 12b, 12c. On the respective transparent electrodes 14a, 14b, 14c, semiconductor photo-active layers 16a, 16b, 16c, are formed, which are composed of amorphous silicon or the like. On the semiconductor photo-active layers 16a, 16b, 16c, back electrodes 18a, 18b, 18c, are formed, the ends of which extend to the adjacent transparent electrodes 14b, 14c, to be connected thereto.
The semiconductor photo-active layers 16a, 16b, 16c, comprise, for example, PIN junctions parallel with the film surface respectively, and when the light enters into them through the glass substrate 10 and the transparent electrodes 14a, 14b, 14c, photovoltages are generated in the respective semiconductor photo-active layers 16a, 16b, 16c. The photovoltages generated in the respective photoelectric conversion cells 12a, 12b, 12c, are added in series because the back electrodes 18a, 18b, 18c, are connected to the adjacent transparent electrodes 14b, 14c.
Normally, in order to manufacture a photovoltaic device of such a structure, a photoetching technique for its micro-workability is employed. In the case of employing the photoetching technique, with reference to the example shown in FIG. 1, a transparent electrode layer is formed on the whole of one main surface of the glass substrate 10, and photo-resist films are formed on the parts corresponding to the semiconductor photo-active layers 16a, 16b, 16c, and then etching is performed and thereafter the photo-resist films are removed, and thereby the semiconductor photo-active layers 16a, 16b, 16c, for the respective photoelectric conversion cells 12a, 12b, 12c, are formed.
Such a photoetching technique excels in micro-workability, but is likely to produce defects in the semiconductor photo-active layer due to pinholes produced in the photo-resist film defining the etching pattern, peeling-off at the fringe of the photo-resist film, etc.
Subsequently, a method not employing photoetching techniques was proposed, for example, in U.S. Pat. No. 4,292,092 issued on Sept. 29, 1981. In this Patent, a laser beam is employed. This method which performs patterning by irradiating the laser beam is extremely effective in that micro-working can be made without employing any wet processing.
However, conventional working by means of laser irradiation has the following problems to be solved. Specifically, the working by the laser beam is essentially a heat working, and therefore if another layer is present under the part of layer to be worked, it is important not to damage it. Otherwise, in addition to burning-off the desired part of the layer, the under layer not required to be burnt-off is also burnt-off, or if not so, it thermally damaged. In U.S. Pat. No. 4,292,092 as cited above, in order to meet this requirement, it is proposed that the laser output or the pulse frequency is selected specifically for each film or layer to be worked.
However, even by this prior art method, the workability is still insufficient because of variations of the film thickness of the semiconductor photo-active layer which is inevitably present. Specifically, the absorption factor of the laser beam varies greatly depending upon the thickness of the film or layer to be worked, and therefore the threshold energy density of the laser for scribing is not always constant. For example, in the case of amorphous silicon, the relationships of absorption factor A, reflection factor R, and transmission factor T of the laser beam to the film thickness are as shown in FIG. 2. As is obvious from FIG. 2, for example, in the case of working amorphous silicon films by a Nd:YAG laser of 10.6 .mu.m wavelength with Q switching, the absorption factor of the laser radiation varies greatly within a range of 5%-20% at a film thickness of 4000 .ANG. or more which is practicable for the photovoltaic device. Accordingly, in the case of working amorphous silicon films by such a YAG laser, when a high laser output is used so as to scribe effectively even if the film thickness is such to give a minimum absorption factor of 5%, a laser beam having an output of four times the threshold energy density is irradiated onto those parts of the film having a thickness corresponding to a maximum absorption factor of 20%. Accordingly, thermal damage to the transparent electrode present under such parts of the amorphous silicon film is unavoidable. Conversely, when a low laser output is employed so that those parts of the film having a thickness corresponding to an absorption factor of 20% can be worked, the laser energy is insufficient at those parts of the film having a thickness corresponding to an absorption factor of 5%. Accordingly, amorphous silicon at those parts is not removed, and remains uncut, resulting in a reduction in output of the photoelectric conversion cell.
Thus, one problem in U.S. Pat. No. 4,292,092 is that since the absorption factor of the laser beam varies greatly depending upon the film thickness of the amorphous silicon film, partial thermal damage is given to the under-layered transparent electrode or the amorphous silicon film at those parts which remain uncut.
U.S. Pat. No. 4,292,092 has another problem, as follows: In general, a metal film such as aluminum is utilized for the back electrode in such a photovoltaic device. FIG. 3 shows the absorption factor A, the reflection factor R, and the transmission factor T in the case of irradiating the laser beam onto such an aluminum back electrode. As is obvious from FIG. 3, 90% or more of the irradiated laser beam is reflected, and therefore a very large laser output is required to scribe it. A laser beam of large output can be used, but the use of a large-output laser beam causes various disadvantages.
For example, as shown in FIG. 4, in the case of a structure wherein the back electrode is separated on the transparent electrode exposed by the semiconductor photo-active layer, for example, the back electrode 18b on the semiconductor photo-active layer 16b is melted due to heating by a laser beam of large output, and a melted part 18ab flows onto the transparent electrode 14b, causing the photoelectric conversion cell 12b to short-circuit. Also, as shown in FIG. 5, in the case of a structure wherein the back electrode is separated on the underlying semiconductor photo-active layer, the parts of the semiconductor photo-active layer 16b bombarded directly by the laser beam of large output are annealed, and the resistance at those parts 16b' is lowered. Consequently, the back electrodes 18a and 18b which may be separated physically from each other are not separated electrically because of the low resistance of the part 16b' of the semiconductor photo-active layer 16b', and accordingly, the open-circuit voltage Voc of the whole photovoltaic device is reduced.
Another laser-beam-applied technique capable of solving one of the problems of U.S. Pat. No. 4,292,092 is disclosed, for example, in U.S. Pat. No. 4,517,403 issued on May 14, 1985. In this Patent, the back electrode of each photo-electric conversion cell is connected in series to the adjacent transparent electrode through silver paste buried in the amorphous silicon. In this Patent, the amorphous silicon layer is not required to be scribed, and therefore the first problem of U.S. Pat. No. 4,292,092 is avoided, namely the problem caused by variation of the laser beam absorption factor due to variation of the film thickness of the amorphous silicon layers. However, this Patent still does not solve the second problem of U.S. Pat. No. 4,292,092, namely scribing of the back electrode.
Also, in copending U.S. Patent Application Ser. No. 749,888 filed June 27, 1985, it is proposed to insert a heat insulating material between the back electrode and the semiconductor photo-active layer in order to remove the deleterious heating effect due to laser-scribing of the back electrode. This Application is characterized in that no damage is given to the underlying semiconductor photo-active layer or the like even when a laser beam of a relatively large output is employed in scribing the back electrode. However, this Application gives no consideration to the change in absorption factor due to the variation of film thickness because the semiconductor photo-active layer itself has been already scribed in the previous process, still leaving the first problem of U.S. Pat. No. 4,292,092.